1. Field of the Invention
The present invention relates to a sheet stacking-aligning apparatus for aligning and stacking sheets, a sheet processing apparatus provided with such sheet stacking-aligning apparatus, and an image forming apparatus provided with such sheet processing apparatus. In particular, the present invention provides a sheet stacking-aligning apparatus with an improved stacking-aligning ability for a sheet bundle, one that is capable of achieving a space saving, a cost reduction and an increase in the capacity of the number of stacked sheets, a sheet processing apparatus provided with such sheet stacking-aligning apparatus, and an image forming apparatus provided with such sheet processing apparatus.
2. Related Background Art
In an image forming apparatus such as a printing press, a copying apparatus or a printer, sheets S subjected to image formation in a main body of the image forming apparatus are temporarily stacked on a process tray 140 in a sheet processing apparatus 100, and there are executed sheet post-processes such as alignment and stapling of the sheet S. Thereafter a bundle is discharged by bundle discharge means 108 onto a stacking tray 400 having an inclined stacking surface as shown in FIG. 10. The discharged sheets S move by its own weight on the inclined stacking surface of the stacking tray 400, and rear ends (trailing edges) of the sheets are aligned on a rear end (trailing edge) aligning wall. The number of stacking sheets S is dependent on a vertically movable stroke of the stacking tray 400.
Also in a sheet processing apparatus as shown in FIG. 12, a bundle of sheets S is conveyed by a gripper 401 of bundle discharge means to a box-shaped stacking tray 400 having a horizontal stacking surface and stacked therein.
Also in a sheet processing apparatus in which a stacking tray 400 has a conventional horizontal stacking surface as shown in FIG. 13, a rotation of a sheet returning paddle 501 causes the sheets S to be stacked with the rear ends thereof aligned.
However, in case of stacking sheets of low rigidity or showing a downward curl on the stacking tray 400 having a conventional inclined stacking surface as shown in FIG. 10, there may result a buckling of the sheets caused by a weight thereof because of a steep inclination, thereby deteriorating the aligning property.
Also in case of stacking stapled sheet bundles S on the stacking tray 400 having the conventional inclined stacking surface as shown in FIG. 11, a rear end of a sheet bundle S may catch on a staple of an already stacked sheet bundle S so that it cannot slide to a rear end aligning wall 70, whereby the stacking property is deteriorated.
Also in the sheet processing apparatus as shown in FIG. 12, the gripper 401 is indispensable as the bundle discharge means, and a combination thereof with the box-shaped stacking tray 400 having a horizontal stacking surface renders the entire apparatus bulky and expensive.
Also in the sheet processing apparatus in which the stacking tray 400 has a horizontal stacking surface as shown in FIG. 13, the alignment by the sheet returning paddle 501 is effective only for the uppermost sheet, whereby the discharge of sheets in a bundle is not possible.